As a conventional scanning system used for TV broadcasting, an interlace scanning system which scans every other horizontal scanning lines has been widely used. In this interlace scanning system, every frame image is formed of a field image consisting of odd-numbered scanning lines and a field image consisting of even-numbered scanning lines, to suppress screen flicker disturbance which causes the entire screen to flicker, thus preventing deterioration of the screen quality.
The interlace scanning system has been adopted as a standard system for television in countries throughout the world. For example, according to PAL (Phase Alternation by Line) system in European television broadcasting, the field frequency is 50 Hz (frame images: 25 frame/second, field images: 50 fields/second).
In particular, the PAL system conventionally adopts a double-speed field frequency system in which the field frequency of inputted image signals is converted to be doubled from 50 Hz to 100 Hz, by performing an interpolation processing or the like, expecting further suppression of the screen flicker disturbance.
FIG. 1 is a block diagram showing a double-speed field conversion circuit 5 using the double-field-frequency system. The double-speed field conversion circuit 5 is integrated in a television receiver 6 which has an input terminal 61, a horizontal/vertical deflection circuit 62, and a CRT 63. This double-speed field conversion circuit 5 has a double-speed converter 51, and a frame memory 52.
The double-speed converter 51 writes image signals of 50 fields/second according to the PAL system inputted from the input terminal 61 into the frame memory 52. Also, the double-speed converter 51 reads the image signals written in the frame memory 52, at a speed twice higher than the writing speed. Thus, the frequency of the image signals of 50 fields/second is converted to a double frequency, so that image signals of 100 fields/second can be generated.
The double-speed converter 51 outputs the image signals subjected to the double conversion to the CRT 63. The CRT 63 displays the inputted image signals on the screen. Horizontal and vertical deflection of the image signals in the CRT 63 is controlled based on a horizontal/vertical rectangular wave which is generated by the horizontal/vertical deflection circuit 62 and has a frequency which is twice that of the inputted image signals.
FIGS. 2A and 2B show a relationship between each field and pixel positions with respect to image signals before and after the double-speed conversion. In each figure, the abscissa axis represents time, and the ordinate axis represents the position of each pixel in the vertical direction. The image signals indicated by white circle marks in FIG. 2A are interlace image signals of 50 fields/second before the double-speed conversion, and the image signals indicated by black circle marks in FIG. 2B are interlace image signals of 100 fields/second after the double-speed conversion.
In the image signals shown in FIG. 2A, fields f1 and f2 are signals generated from one single unit-frame of a film. Likewise, fields f3 and f4 constitute one single unit-frame. Since these image signals are interlace image signals, the pixel positions in the vertical direction differ between adjacent fields. Therefore, it is impossible to create a new field between every two adjacent fields, maintaining the characteristics of interlacing.
Hence, as shown in FIG. 2B, two fields f2′ and f1′ are newly generated between the fields f1 and f2. No new fields are generated between the fields f2 and f3 but two new fields f4′ and f3′ are generated between the fields f3 and f4. That is, one unit-frame is formed of four fields forming two frames.
In some cases, those newly generated fields f1′, f2′, . . . are obtained by using a median filter or the like, supposing that each pixel value is an intermediate value among three pixels surrounding each pixel. The newly generated fields f1′, f2′, . . . have the same contents as the fields f1, f2, . . . , respectively.
Specifically, the double-speed field conversion circuit 5 provides parts in each of which two new fields are generated and parts in each of which no new fields are generated, alternately among fields of image signals before the double-speed conversion. The number of screen images per unit time can thus be increased so that the screen flicker disturbance as previously described can be suppressed.
In order to watch a cinema film consisting of still images of 24 unit-frames/second on an ordinary TV set, television-to-cinema conversion (which will be hereinafter referred to as telecine conversion) is carried out to attain interlace television signals. FIGS. 3A and 3B show a relationship between each field and an image position in case where an image moves in the horizontal direction, with respect to the image signals after the telecine conversion. The abscissa axis represents the position of the image in the horizontal direction, and the ordinate axis represents time. In the image signals before the double-speed conversion shown in FIG. 3A, the fields f1 and f2 form one single unit-frame, so that the image is displayed at the same position. This image moves in the horizontal direction (to the right side) as the field shifts to the field f3. Since the field f4 forms part of the same unit-frame as the field f3, the image is displayed at the same position as in the field f3.
If image signals shown in FIG. 3A after the telecine conversion are subjected to the double-speed conversion according to the double-speed field frequency system, as shown in FIG. 3B, an equal image is displayed at an equal position in the fields f1, f2′, f1′, and f2 forming one single unit-frame. Similarly, as equal image is displayed at an equal position in the fields f3, f4′, f3′ and f4 forming one single unit-frame.
The image signals subjected to the double-speed conversion after the telecine conversion are displayed at one equal position from the field f1 to the field f2 as shown in FIG. 3B. On the other side, the signals greatly move in the horizontal direction when the field shifts from f2 to f3. In particular, the image signals after the double-speed conversion form fields regularly at a cycle of one field per 1/100 second. Therefore, a time band in which an image moves is shorter than another time band in which an image stands still. When a program is actually watched by a CRT, motions of images look discontinuous. To eliminate such discontinuity in motions, for example, a screen image is divided into blocks each consisting of predetermined pixels, based on a block matching method, and a motion vector is obtained by evaluating similarity in units of blocks. The motion is corrected by shifting pixel positions for every block in accordance with the obtained motion vector.
Suppose, however, a case that motions take place in two directions on a television screen T, as shown in FIG. 4. That is, an image A of an object moves to the left side in the direction of an arrow X1 in the figure while a background B moves to the right side in the direction of an arrow X2 in the figure. In this case, the direction of a motion vector as described above cannot be detected correctly, and therefore, the discontinuity in motions of images as described above cannot be eliminated. The reference symbol C in FIG. 4 denotes a block to perform the block matching described above.